The present invention pertains generally to systems and methods for cryoablating internal tissue. More particularly, the present invention pertains to systems and methods for cryoablating conduction blocks to treat patients experiencing heart arrhythmias such as atrial fibrillation. The present invention is particularly, but not exclusively, useful for ablating a substantially circumferentially shaped portion of tissue surrounding the ostium of a pulmonary vein in a single step.
Atrial fibrillation is an irregular heart rhythm that adversely affects approximately 2.5 million people in the United States. It is believed that at least one-third of all atrial fibrillation originates near the ostium of the pulmonary veins. Anatomically, two pairs of pulmonary veins are connected to the left atrium of the heart with each pair delivering blood to the heart from one of the patient""s lungs. It is further believed that the optimal technique to treat atrial fibrillation is to create circumferential lesions around the ostia where a pulmonary vein connects with the left atrium. More specifically, the goal is to ablate tissue to form a conduction block to thereby prohibit the transmission of Irregular electrical signals that can cause an arrhythmia. To be effective, the conduction block must completely block Irregular signals and this often requires the ablation of a relatively deep, uniform lesion.
Heretofore, due to the relatively large diameters of these ostia, cryoablation procedures have required multiple, successive contacts between the cryo-element and the tissue around the periphery of an ostium. More specifically, these procedures have required the cryo-element to be successively moved around the ostia to create a patchwork array of ablations. This often results In a non-uniform circumferential ablation that fails to form an adequate conduction block. Furthermore, when multiple, successive contacts are prescribed, special catheter structures are generally required to give a catheter the agility required to carefully move from one location to the next within the pulmonary vein. These structures increase the size of the distal end of the catheter, making the catheter harder to steer and navigate through the vasculature of the patient to the treatment site. In short, procedures requiring multiple contacts tend to be complicated, time consuming, difficult to perform, and are generally unreliable.
Another factor that must be considered when ablating internal tissue is the stability of the ablation element (e.g. cryo-element) relative to the target tissue. During ablation, movements of the patient such as heartbeats and breathing can cause the ablation element to move or bounce. Failure to prevent these movements of the ablation element relative to the target tissue can disrupt the flow of energy between the ablation element and the tissue resulting in a non-uniform ablation. As indicated above, non-uniform ablations often result in an ineffective conduction block.
In light of the above, it is an object of the present invention to provide systems and methods suitable for the purposes of cryoablating substantially circumferential ablations of internal tissue in a single step. It is another object of the present invention to provide systems and methods for forming conductive blocks to treat heart arrhythmias such as atrial fibrillation. It is yet another object of the present invention to provide systems and methods for cryoablating internal target tissue that can be performed quickly and are relatively reliable. Still another object of the present invention is to provide systems and methods for cryoablating circumferential ablation that are easy to use or perform and are comparatively cost effective.
The present invention is directed to a system and method for cryoablating internal target tissue at a treatment site. In one application of the system and method, a substantially circumferential portion of tissue surrounding the ostium of a pulmonary vein is ablated. The resulting lesion functions as a conduction block to treat heart arrhythmias such as atrial fibrillation.
For the present invention, the system includes a balloon that is mounted on the distal end of a balloon catheter. The balloon catheter is elongated and defines a longitudinal axis in the direction of elongation. In more detail, the balloon catheter is tubular shaped and formed with a lumen that extends between the proximal and distal ends of the balloon catheter. The balloon is attached to the distal end of the balloon catheter and placed in fluid communication with the lumen of the balloon catheter. With this combination of structure, a saline solution can be introduced into the balloon by pumping the saline solution into the proximal end of the balloon catheter from an extracorporeal location. In greater structural detail, the balloon has a substantially annular shaped cross-section in a plane substantially orthogonal to the longitudinal axis of the balloon catheter.
The system further includes a cryo-catheter that is disposed within the lumen of the-balloon catheter. The cryo-catheter extends between a distal end and a proximal end and surrounds a lumen for the cryo-catheter. In one implementation, the balloon catheter and cryo-catheter are arranged to be co-axial about the longitudinal axis of the balloon catheter. The system also includes a cryo-element that is mounted on the cryo-catheter at the cryo-catheter""s distal end. In one implementation, the cryo-element is formed with an expansion chamber that is placed in fluid communication with the lumen of the cryo-catheter when the cryo-element is mounted on the cryo-catheter.
The cryo-catheter can further include a supply tube that is positioned inside the lumen of the cryo-catheter. In one implementation, the supply tube is positioned inside the lumen of the cryo-catheter to establish a return line between the inner surface of the cryo-catheter and the outer surface of the supply tube. Furthermore, the supply tube can extend from the proximal end to the distal end of the cryo-catheter.
The system further includes a refrigerant supply unit that is positioned at an extracorporeal location to introduce a fluid refrigerant into the proximal end of the supply tube. The fluid refrigerant then traverses through the lumen of the supply tube and exits the supply tube into the expansion chamber of the cryo-element. In one implementation, a flow restricting device such as a capillary tube can be used to restrict flow at the distal end of the supply tube. In this implementation, the fluid refrigerant passes through the restriction and then expands into the chamber to cool the cryo-element. In a particular embodiment of the present invention, a fluid refrigerant is used that transitions from a liquid state to a gaseous state as it expands into the cryo-element chamber. Heat absorbed by the refrigerant during this phase transition (i.e. latent heat) cools the cryo-element. After expansion, the gaseous fluid refrigerant can pass through the return line and exit the patient at the proximal end of the cryo-catheter.
In operation, the cryo-element is inserted Into the vasculature of the patient and advanced within the vasculature using the cryo-catheter until the cryo-element is positioned at the treatment site, To facilitate positioning of the cryo-element at the treatment site, the distal portion of cryo-catheter can be formed as an articulation segment (see more detailed description below). With the cryo-element in place, the balloon catheter is then used to advance the annular shaped balloon over the cryo-catheter to the treatment site. At the treatment site, the annular shaped balloon is interposed between the cryo-element and the target tissue.
In an alternative implementation of the system, a guidewire can be used to position the cryo-element and balloon at the treatment site. In this implementation, the tip of a guidewire is first inserted into the vasculature of the patient and advanced past the target tissue. Next, an eyelet mounted on the balloon catheter is threaded onto the guidewire and the balloon catheter and cryo-catheter are advanced within the vasculature of the patient until the cryo-element is located at the treatment site. At the treatment site, the annular shaped balloon can be moved relative to the cryo-element to interpose the balloon between the cryo-element and the target tissue.
With the balloon interposed between the cryo-element and the target tissue, saline solution Is pumped into the balloon causing the balloon to expand. More specifically, an inner surface portion of the balloon expands toward the cryo-element and an outer surface portion of the balloon expands toward the target tissue. Filling of the balloon with saline solution is continued until the expanded balloon contacts both the cryo-element and the surrounding target tissue. The shape of the balloon (i.e. the annular shape) allows the balloon to surround the cryo-element and provide a large contact area between the balloon and the cryo-element. The large contact area, in turn, provides for good heat transfer between the saline solution and the cryo-element. In addition, the expanded balloon functions to anchor the cryo-element in place at the site of the target tissue.
Once the balloon has been adequately filled with liquid, the refrigerant supply unit is activated to introduce a fluid refrigerant into the expansion chamber of the cryo-element and thereby cool the cryo-element. In one implementation, nitrous oxide is used as the refrigerant allowing the cryo-element to be cooled to a temperature of approximately xe2x88x9285 degrees Celsius. The cooling of the cryo-element, in turn, freezes and cools the liquid in the balloon to a temperature of approximately xe2x88x9285 degrees Celsius. The resulting xe2x80x9cice ballxe2x80x9d extracts heat from surrounding tissue resulting in the cryoablation of a substantially circumferential portion of tissue.
The system can also include a subsystem for directing energy into the xe2x80x9cice ballxe2x80x9d to quickly thaw the frozen xe2x80x9cice ballxe2x80x9d and restore blood flow through the affected conduit (e.g. pulmonary vein). Once the xe2x80x9cice ballxe2x80x9d is thawed, the saline solution can be removed from the balloon and the balloon withdrawn from the patient""s body. In one embodiment of the present invention, a radiofrequency (rf) antenna is mounted on either the first or cryo-catheter to thaw the xe2x80x9cice ballxe2x80x9d and facilitate removal of the balloon from the patient.